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+(ns max-subarray

+  (:require [clojure.test :refer [deftest is]]))

+

+(defn propagating

+  "Transducer that emits the current maximum value, according to cmp"

+  [init cmp]

+  (fn [rf]

+    (let [cur-max (volatile! init)]

+      (completing

+       (fn [acc nxt]

+         (vswap! cur-max cmp nxt)

+         (rf acc @cur-max))))))

+

+(defn max-sequence [xs]

+  (-> (into [0]

+            (comp (propagating 0 (comp #(max 0 %) +)) ;; Maintain the running sum, as long as it's non-zero

+                  (propagating 0 max))                ;; Emit the current maximum running sum

+            xs)

+      peek)) ;; Get the last maximum running sum (peek on vector makes this fast)

+

+(deftest simple

+  (is (= (max-sequence  [-2, 1, -3, 4, -1, 2, 1, -5, 4]) 6))

+  (is (= (max-sequence  []) 0))

+  (is (= (max-sequence  [1 2 3 4]) 10))

+  (is (= (max-sequence  [-1 -2 -3 -4]) 0))

+

+  )

+

+(defn collect-runs [eq]

+  (let [sentinel (Object.)]

+    (fn [rf]

+      (let [c-v (atom sentinel)

+            current (atom [])]

+        (fn

+          ([] (rf))

+          ([acc] (rf (rf acc @current)))

+          ([acc nxt]

+           (when (= @c-v sentinel)

+             (reset! c-v nxt))

+           (let [result (if (eq @c-v nxt)

+                          (do (swap! current conj nxt)

+                              acc)

+                          (let [r (rf acc @current)]

+                            (reset! current [nxt])

+                            r))]

+             (reset! c-v nxt)

+             result)))))))

+

+(defn series-sum [n]

+  (format "%.2f" (double (apply + (map #(/ 1.0 (doto % prn)) (take n (iterate #(+ % 3.0) 1.0)))))))